2 Answers
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A parity block is used by certain RAID levels. Redundancy is achieved by the use of parity blocks. If a single drive in the array fails, data blocks and a parity block from the working drives can be combined to reconstruct the missing data.

Given the diagram below, where each column is a disk, assume A1 = 00000111, A2 = 00000101, and A3 = 00000000. Ap, parity block generated by XORing A1, A2, and A3, will then equal 00000010. If the Disk 1 fails, A2 will no longer be accessible, but can be reconstructed by XORing A1, A3, and Ap:

Parity is used to not only help maintain data transmission in raid arrays they are also used to reconstruct data if Disk 0, 1, or 2 goes down. I think that this image is a better representation of how raid three looks like.

In the image:
block 1a, 1b, 1c are duplicated data the first parity keeps track of the data blocks